Evaporative emission control systems are widely used in Internal Combustion Engine (ICE) powered motor vehicles to prevent evaporative fuel, i.e. fuel vapor, from being emitted from the fuel tank into the atmosphere. There are generally three main components that control such evaporative emission operations: carbon canister vent valves and canister purge valves (both vacuum-operated and electronically operated). One or more of the above components may typically be found in an ICE powered motor vehicle to control evaporative emission.
The most common valve used to control evaporative emission operation is the canister purge solenoid, which is a normally closed solenoid that is mounted in line between a carbon canister and the intake manifold of an internal combustion engine. In operation, when the Electronic Engine Control (EEC) assembly energizes the solenoid, the solenoid opens, thus allowing the intake manifold vacuum to draw fuel vapors from the canister into the cylinders for combustion. In contrast, when the electronic control assembly de-energizes the solenoid, fuel vapors are stored in the carbon canister.
As readily seen, in the event that one or more of the above evaporative emission control components malfunctions, fuel vapors may be vented improperly resulting in reduced engine performance and possible release of vapors into the atmosphere. It is thus desirable to employ an on-board diagnostic system capable of detecting deficiencies in evaporative vapor emission control components and identifying such deficiencies so that corrective measures may be taken.
One such attempt is disclosed in U.S. Pat. No. 5,085,197, issued to Mader et al, entitled "Arrangement for the Detection of Deficiencies in a Tank Ventilation System." As reflected in the title, the U.S. Pat. No. 197 discloses a system for the detection of defects in a tank ventilation system formed of a fuel tank, an active carbon filter, a control unit, a lambda probe, a tank ventilation valve (canister purge solenoid), and a flow sensor. In operation, the control unit examines signals arriving from the lambda probe and from the flow sensor along with the outgoing tank ventilation control signals to generate an error signal indicative of a defect in the ventilation system such as a malfunctioning valve and/or a leak in one or more of the connecting hoses.
Significantly, the U.S. Pat. No. 197 discloses a ceramic PTC resistor for use as a flow sensor. Such resistors are particularly known to be affected by changes in temperature gradient, engine load and engine speed. While the U.S. Pat. No. 197 does disclose temperature compensation means to ensure readings are taken under similar ambient conditions, the remaining engine parameters are neither addressed nor compensated for. As a result, variations in engine speed or engine load may substantially alter the information received and thus produce a false or erroneous error signal.